Abstract:

Some implementations of the invention provide hidden static images. Some
such images can only be perceived when viewed from an angle to the normal
to a surface. Such images may appear as a solid color when viewed along
an axis perpendicular to a surface, but may reveal a hidden image when
viewed from an angle relative to that axis. The hidden image may be
defined according to interfaces between areas that pass substantially the
same wavelength when viewed along the axis, but which pass noticeably
different wavelengths when viewed from an angle relative to that axis.
The hidden images may or may not be discernable to a human observer. The
hidden image may comprise a code that is not readable by an optical
scanner positioned to read along an axis normal to a surface, but that is
readable by an optical scanner positioned to read along another axis.

Claims:

1. An apparatus, comprising:a substantially transparent layer;a reflective
layer;a first spacer disposed between a first portion of the
substantially transparent layer and a first portion of the reflective
layer, the first spacer having a first index of refraction and a first
thickness; anda second spacer disposed proximate the first spacer and
between a second portion of the substantially transparent layer and a
second portion of the reflective layer, the second spacer having a second
index of refraction and a second thickness;wherein a first cavity
comprising the first spacer, the reflective layer and the substantially
transparent layer passes a first wavelength of reflected light when
viewed along an axis substantially normal to the first portion of the
reflective layer, the first cavity passing a second wavelength of
reflected light and attenuating the first wavelength of reflected light
when viewed from an angle θ to the axis; andwherein a second cavity
comprising the second spacer, the reflective layer and the substantially
transparent layer passes the first wavelength of reflected light when
viewed along the axis, the second cavity passing a third wavelength of
reflected light and attenuating the first wavelength of reflected light
when viewed from the angle θ to the axis.

2. The apparatus of claim 1, wherein the reflective layer is
discontinuous.

3. The apparatus of claim 1, further comprising an absorber layer disposed
between the first spacer and the substantially transparent layer, the
absorber layer being further disposed between the second spacer and the
substantially transparent layer.

4. The apparatus of claim 1, wherein a difference between the second
wavelength and the third wavelength is perceivable as a chromatic
difference to a human observer.

5. The apparatus of claim 1, wherein an interface is formed by a first
edge of the first spacer that is proximate a second edge of the second
spacer, the interface not being discernable to a human observer when
viewed along the axis, but being discernable to the human observer when
viewed from the angle θ to the axis.

6. The apparatus of claim 1, wherein an interface is formed by a first
edge of the first spacer that is proximate a second edge of the second
spacer, wherein the interface forms a portion of a code that is readable
by an optical scanner when the optical scanner is positioned to read at
the angle θ to the axis, wherein the code is not readable by the
optical scanner when the optical scanner is positioned to read along the
axis.

7. The apparatus of claim 1, wherein at least one of the first wavelength,
the second wavelength or the third wavelength is outside a range of
wavelengths perceivable to a human observer.

8. The apparatus of claim 5, wherein the interface forms at least part of
an image that is visible to a human observer when viewed from the angle
θ to the axis but not visible to the human observer when viewed
along the axis.

9. An apparatus, comprising:a substantially transparent layer;a reflective
layer;first means for passing a first peak wavelength of light when
viewed along an axis perpendicular to a first portion of the reflective
layer and for passing a second peak wavelength of light when viewed from
an angle θ to the axis; andsecond means for reinforcing passing the
first peak wavelength of light when viewed along the axis and for passing
a third peak wavelength of light when viewed from the angle θ to
the axis.

10. The apparatus of claim 9, wherein a difference between the second peak
wavelength and the third peak wavelength is perceivable as a chromatic
difference to a human observer.

11. The apparatus of claim 9, wherein an interface is formed by a first
edge of the first means that is proximate a second edge of the second
means, the interface being discernable to a human observer when viewed
from the angle θ to the axis but not being discernable to a human
observer when viewed along the axis.

12. The apparatus of claim 9, wherein an interface is formed by a first
edge of the first means that is proximate a second edge of the second
means, wherein the interface forms a portion of a code that is readable
by an optical scanner when the optical scanner is positioned to read at
the angle θ but not readable by the optical scanner when the
optical scanner is positioned to read along the axis.

13. The apparatus of claim 9, wherein at least one of the first peak
wavelength, the second peak wavelength or the third peak wavelength is
outside a range of wavelengths perceivable to a human observer.

14. The apparatus of claim 11, wherein the interface forms at least part
of an image that is visible to a human observer when viewed from the
angle θ but not visible to a human observer when viewed along the
axis.

15. A method, comprising:forming an absorber layer on a substantially
transparent sheet;depositing a first material on the absorber layer in a
first area;depositing a second material on the absorber layer in a second
area proximate the first area;forming a first reflective surface on the
first material; andforming a second reflective surface on the second
material,wherein the first material has a first index of refraction and a
first thickness and wherein the absorber layer, the first material and
the first reflective surface form a first cavity configured to pass a
first wavelength of light when viewed along an axis perpendicular to the
first reflective surface and to pass a second wavelength of light and
attenuate the first wavelength of light when viewed from an angle θ
to the axis; andwherein the second material has a second index of
refraction and a second thickness and wherein the absorber layer, the
second material and the second reflective surface form a second cavity
configured to pass the first wavelength of light when viewed along the
axis and to pass a third wavelength of light and attenuate the first
wavelength of light when viewed from the angle θ to the axis.

16. The method of claim 15, wherein the first reflective surface and the
second reflective surface are continuous.

17. The method of claim 15, wherein the first reflective surface and the
second reflective surface are discontinuous.

18. A method, comprising:forming a first reflective surface on a first
area of a substrate;forming a second reflective surface on a second area
of the substrate proximate the first reflective surface;depositing a
first material on the first reflective surface;depositing a second
material on the second reflective surface;applying a first absorber on
the first material; andapplying a second absorber on the second
material,wherein the first material has a first index of refraction and a
first thickness and wherein the absorber layer, the first material and
the first reflective surface form a first cavity configured to pass a
first wavelength of light when viewed along an axis perpendicular to the
first reflective surface and to pass a second wavelength of light and
attenuate the first wavelength of light when viewed from an angle θ
to the axis; andwherein the second material has a second index of
refraction and a second thickness and wherein the absorber layer, the
second material and the second reflective surface form a second cavity
configured to pass the first wavelength of light when viewed along the
axis and to pass a third wavelength of light and attenuate the first
wavelength of light when viewed from the angle θ to the axis.

19. The method of claim 18, wherein forming the first reflective surface
and the second reflective surface comprises forming a continuous
reflective layer on the first area and the second area of the substrate.

20. The method of claim 18, wherein applying the first absorber and the
second absorber comprises applying a continuous absorber layer on the
first material and the second material.

Description:

[0002]Static images, which may include graphic images, patterns, text,
codes and the like, have many uses. Images and patterns may be purely
decorative, or they may be associated with a product, a brand name, etc.
Textual images may convey various types of information. Codes may be used
in various contexts. Bar codes, for example, are now widely used for
identifying product types, tracking inventory and the like. Although
existing methods for fabricating images may be generally satisfactory, it
would be desirable to provide improved methods and devices for making and
using static images.

SUMMARY

[0003]Some implementations provide hidden static images. Some such images
can only be perceived when viewed from an angle to the normal of a
surface. For example, some such images appear as a solid color when
viewed along an axis perpendicular to a surface, but reveal a hidden
image when viewed from an angle relative to that axis. The hidden image
may be defined according to interfaces between optical cavities that pass
substantially the same wavelength when viewed along the axis
perpendicular to the surface, but which pass noticeably different
wavelengths and attenuate other wavelengths when viewed from an angle
relative to that axis. Alternative implementations may involve optical
cavities configured to pass noticeably different wavelengths when viewed
along the axis perpendicular to the surface and to pass substantially the
same wavelength when viewed from an angle relative to that axis.

[0004]The hidden images may or may not be discernable to a human observer.
The hidden image may comprise a code that is not readable by an optical
scanner when it is positioned to read along an axis normal to a surface,
but that is readable by the optical scanner when it is positioned to read
along another axis at an angle to normal.

[0005]Some embodiments provide an apparatus that includes a substantially
transparent layer, a reflective layer, a first spacer having a first
index of refraction and a first thickness, and a second spacer having a
second index of refraction and a second thickness. The first spacer may
be disposed between a first portion of the substantially transparent
layer and a first portion of the reflective layer. The second spacer may
be disposed proximate the first spacer and between a second portion of
the substantially transparent layer and a second portion of the
reflective layer.

[0006]A first cavity comprising the first spacer, the reflective layer and
the substantially transparent layer may pass a first wavelength of
reflected light when viewed along an axis substantially normal to the
first portion of the reflective layer. The first cavity may pass a second
wavelength of reflected light and attenuate the first wavelength of
reflected light when viewed from an angle θ to the axis.

[0007]A second cavity comprising the second spacer, the reflective layer
and the substantially transparent layer may pass the first wavelength of
reflected light when viewed along the normal axis. The second cavity may
pass a third wavelength of reflected light and attenuate the first
wavelength of reflected light when viewed from the angle θ to the
normal axis.

[0008]The reflective layer may be continuous or discontinuous, according
to the implementation. The apparatus may further comprise an absorber
layer disposed between the first spacer and the substantially transparent
layer. An absorber layer may be further disposed between the second
spacer and the substantially transparent layer.

[0009]A difference between the second wavelength and the third wavelength
may be perceivable as a chromatic difference to a human observer and/or
to a machine.

[0010]An interface may be formed by a first edge of the first spacer that
is proximate a second edge of the second spacer. The interface may not be
discernable to an observer when viewed along the axis, but may be
discernable to the observer when viewed from the angle θ to the
axis. The interface may form at least part of an image that is visible to
a human observer when viewed from the angle θ to the axis but not
visible to the human observer when viewed along the axis. The interface
may form a portion of a code that is readable by an optical scanner when
the optical scanner is positioned to read at the angle θ to the
axis. However, the code may not be readable by the optical scanner when
the optical scanner is positioned to read along the axis. At least one of
the first wavelength, the second wavelength or the third wavelength may
be outside a range of wavelengths perceivable to a human observer.

[0011]Other embodiments provide an apparatus that includes these elements:
a substantially transparent layer; a reflective layer; a first cavity
configured for passing a first peak wavelength of light when viewed along
an axis perpendicular to a first portion of the reflective layer and for
passing a second peak wavelength of light when viewed from an angle
θ to the axis; and a second cavity configured for passing the first
peak wavelength of light when viewed along the axis and for passing a
third peak wavelength of light when viewed from the angle θ to the
axis.

[0012]The difference between the second peak wavelength and the third
wavelength may be perceivable as a chromatic difference to an observer,
e.g., to a human observer. However, at least one of the first peak
wavelength, the second peak wavelength or the third peak wavelength may
be outside a range of wavelengths perceivable to a human observer.

[0013]An interface may be formed by a first edge of the first means that
is proximate a second edge of the second means. The interface may be
discernable to an observer when viewed from the angle θ to the
axis, but may not be discernable to the observer when viewed along the
axis. The interface may form at least part of an image that is visible to
an observer when viewed from the angle θ but not visible to the
human observer when viewed along the axis. The interface may form a
portion of a code that is readable by an optical scanner when the optical
scanner is positioned to read at the angle θ but that is not
readable by the optical scanner when the optical scanner is positioned to
read along the axis.

[0014]Some methods described herein involve the following: forming an
absorber layer on a substantially transparent sheet; depositing a first
material on the absorber layer in a first area; depositing a second
material on the absorber layer in a second area proximate the first area;
forming a first reflective surface on the first material; and forming a
second reflective surface on the second material. The first reflective
surface and the second reflective surface may be continuous or
discontinuous, according to the implementation.

[0015]The first material has a first index of refraction and a first
thickness. The absorber layer, the first material and the first
reflective surface may form a first cavity configured to pass a first
wavelength of light when viewed along an axis perpendicular to the first
reflective surface and to pass a second wavelength of light, and
attenuate the first wavelength of light, when viewed from an angle
θ to the axis.

[0016]The second material has a second index of refraction and a second
thickness. The second index of refraction and the second thickness may or
may not be different from the first index of refraction and the first
thickness. The absorber layer, the second material and the second
reflective surface may form a second cavity configured to pass the first
wavelength of light when viewed along the axis and to pass a third
wavelength of light, and attenuate the first wavelength of light, when
viewed from the angle θ to the axis.

[0017]Alternative methods may involve the following: forming a first
reflective surface on a first area of a substrate; forming a second
reflective surface on a second area of the substrate proximate the first
reflective surface; depositing a first material on the first reflective
surface; depositing a second material on the second reflective surface;
applying a first absorber on the first material; and applying a second
absorber on the second material.

[0018]The first material has a first index of refraction and a first
thickness. The absorber layer, the first material and the first
reflective surface may form a first cavity configured to pass a first
wavelength of light when viewed along an axis perpendicular to the first
reflective surface and to pass a second wavelength of light, and
attenuate the first wavelength of light, when viewed from an angle
θ to the axis.

[0019]The second material has a second index of refraction and a second
thickness. The second index of refraction and the second thickness may or
may not be different from the first index of refraction and the first
thickness. The absorber layer, the second material and the second
reflective surface may form a second cavity configured to pass the first
wavelength of light when viewed along the axis and to pass a third
wavelength of light, and attenuate the first wavelength of light, when
viewed from the angle θ to the axis.

[0020]Forming the first reflective surface and the second reflective
surface may involve forming a continuous reflective layer or a
discontinuous reflective layer on the first area and the second area of
the substrate. Applying the first absorber and the second absorber
comprises applying a continuous absorber layer or a discontinuous
absorber layer on the first material and the second material.

[0021]Some embodiments of the present invention provide hardware that is
configured to perform the methods of the invention. Some implementations
of the invention provide software stored on computer-readable media, the
software including instructions for controlling devices to perform these
and other methods. These and other features of the present invention will
be presented in more detail in the following detailed description of the
invention and the associated figures.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022]FIG. 1A illustrates a sheet having a hidden image formed thereon
being viewed from two positions.

[0023]FIG. 1B illustrates what a viewer would observe when observing the
sheet of FIG. 1A from the two positions indicated.

[0024]FIG. 2A illustrates one example of thin film stacks that may be used
to form a portion of a hidden image.

[0025]FIG. 2B illustrates another example of thin film stacks that may be
used to form a portion of a hidden image.

[0026]FIG. 3 is a graph of viewing angle versus peak wavelength for two
examples of spacer material in the thin film stacks of FIGS. 2A and/or
2B.

[0027]FIG. 4 is a graph of viewing angle versus peak wavelength for other
examples of spacer material in the thin film stacks of FIGS. 2A and/or
2B.

[0028]FIG. 5 is a graph of viewing angle versus peak wavelength for still
other examples of spacer material in the thin film stacks of FIGS. 2A
and/or 2B.

[0029]FIG. 6A illustrates a sheet having a hidden image formed thereon
being scanned by an optical scanner from two positions.

[0030]FIG. 6B illustrates what the optical scanner would detect when
scanning the sheet of FIG. 6A from the two positions indicated.

[0031]FIG. 7 is a flow chart that outlines steps of forming a stack that
includes a hidden image.

[0033]FIG. 9A illustrates yet another example of thin film stacks that may
be used to form a portion of a hidden image.

[0034]FIG. 9B illustrates another example of thin film stacks that may be
used to form a portion of a hidden image.

[0035]FIG. 10 is a flow chart that outlines steps of forming the thin film
stacks of FIG. 9A or 9B according to some implementations of the
invention.

DETAILED DESCRIPTION

[0036]In this application, numerous specific details are set forth in
order to provide a thorough understanding of the present invention.
However, the present invention may be practiced without some or all of
these specific details. In some instances, well known process steps have
not been described in detail.

[0037]One example of a hidden image according to some implementations will
now be described with reference to FIGS. 1A and 1B. When sheet 101 is
viewed from position 105, along axis 101 that is perpendicular to sheet
101, no image is apparent. This condition is shown by the apparently
blank sheet 101a in FIG. 1B.

[0038]However, when sheet 101 is viewed from position 115, along axis 120
that is θ degrees from axis 110, hidden image 125 may be seen by a
viewer or imaging device. Hidden image 125 is revealed to the viewer and
may appear to the viewer as depicted on sheet 101b of FIG. 1B. In this
example, areas 130 appear as a first color, areas 135 appear as a second
color and background area 140 appears as a third color. Interfaces 145
between these areas define hidden image 125.

[0039]FIG. 2A provides one example of thin film stacks that may be used to
produce a portion of a hidden image. Stack 200 includes substrate 205,
which is formed of substantially transparent material such as glass,
plastic, etc.

[0040]Spacer 1 has an index of refraction n1 and spacer 2 has an
index of refraction n2. Spacers 1 and 2 may be formed of various
substances, such as air, transmissive dielectrics, etc. Some examples of
the latter include, but are not limited to, SiO2, AlOx,
SiO2 and Si3N4. In some implementations, n1 differs
substantially from n2.

[0041]However, in some embodiments n1 is substantially the same as
n2. In some such embodiments, spacers 1 and 2 are formed from the
same material, but each has a different thickness. Examples of some such
embodiments are described below with reference to FIG. 9B.

[0042]Reflectors 215a and 215b are formed of a highly reflective material,
such as aluminum, gold, tin, silver, or other metals, and may be formed
of or include dielectric mirrors. In this example, reflectors 215a and
215b are discontinuous. However, in some such implementations (such as
the embodiment depicted in FIG. 2B), reflective layer 215 may be
continuous.

[0043]Absorber 210 is partially reflective and partially transmissive and
partially absorptive. The thickness and material for absorber 210 are
chosen so that absorber 210 allows ambient light received from substrate
205 to pass through spacers 1 and 2 and vice versa, but yet reflects a
portion of the light that has reflected from reflectors 215a and 215b.
Absorber 210 may be formed, for example, from MoCr ("molychrome," an
alloy of molybdenum and chromium), Cr, Ti, Ta, W, or any other suitable
material.

[0044]Some embodiments do not include an absorber layer. For example, some
embodiments provide dichroic stacks, which are interferometric stacks
with no absorbers.

[0045]However, in this example, a first optical cavity is formed by
reflector 215a, spacer 1 and absorber 210. In this example, the first
optical cavity corresponds with a portion of area 135 of FIG. 1B. A
second optical cavity is formed by reflector 215b, spacer 2 and absorber
210. Here, the second optical cavity corresponds with a portion of area
130 of FIG. 1B. Interface 145 delineates a separation zone between area
130 and area 135. In some embodiments, the separation zone is sharp such
that there is little or no overlap between area 130 and area 135, while
in other embodiments the separation zone is diffuse such that there is a
gradual transition between area 130 and area 135.

[0046]As in FIGS. 1A and 1B, a comparison will be made in various
embodiments between what is observed from position 105, along axis 110
that is perpendicular to reflector 215b, and what is observed from
position 115, along axis 120 that is at least 0 degrees from axis 110. As
shown below with respect to FIGS. 3-5 there is a range of angles or
viewing cone within which an image may be perceived: below a certain
theta the image is hidden, within a certain range of thetas the image is
apparent, but above a certain theta the image is not viewable (e.g., as
theta approaches 90 degrees). For example, as described in more detail
below, in some implementations a chromatic difference between areas of a
hidden image may not be discernable when viewed along an axis parallel to
axis 110, but may be discernable when viewed along an axis parallel to
axis 120.

[0047]Positions 105 and 115 and the corresponding axes 110 and 120 are
shown separately in FIG. 2A from positions 105' and 115' and the
corresponding axes 110' and 120'. This distinction is made in FIG. 2A for
the purpose of illustrating how light is affected by traveling through
spacer 1 and spacer 2 along different paths. Axis 110' is perpendicular
to reflector 215a and axis 120' is θ degrees from axis 110'. In
this example, axis 110 is substantially parallel to axis 110', though
this is not necessarily true in all implementations and/or viewing
distances.

[0048]The reader should bear in mind, however, that FIG. 2A is not drawn
to scale. In practice, positions 105 and 115 of the viewer may be much
further away from stack 200 than is suggested by the scale of FIG. 2A.

[0049]As depicted in FIG. 1A, at such distances, an observer at position
105 could simultaneously view light that is arriving along axis 110 and
axis 110'. Similarly, an observer at position 115 could simultaneously
view light that is arriving along axis 120 and axis 120'.

[0050]Preferably, the materials and/or thicknesses of spacers 215 and 220
are selected such that λd1, the peak wavelength reinforced by
the first optical cavity that includes spacer 1, is approximately the
same as λd2, the peak wavelength reinforced by the second
optical cavity that includes spacer 2, for light that is reflecting along
axis 110 and axis 110'. In some such implementations, areas 130 and 135
will appear to be approximately the same color when observed along axes
110 and 110'. Interface 145 will not be distinguishable in some
embodiments.

[0051]The materials and/or thicknesses of spacers 215 and 220 are selected
such that λd1 is substantially different from λd2
for light that is reflecting along axis 120 and axis 120'. In some such
implementations, a human observer will detect a chromatic difference
between areas 130 and 135 when viewing stack 200 along axis 120 and axis
120', and interface 145 will be distinguishable. As the viewing angle
changes, the peak wavelength that is reinforced by the first optical
cavity changes at a different rate from the peak wavelength that is
reinforced by the second optical cavity.

[0052]As shown in FIG. 2B, in some implementations this change may be
produced entirely by a difference in refractive indexes between spacer 1
and spacer 2. In such implementations, the thickness of spacer 1 and
spacer 2 may be substantially the same.

[0053]FIG. 3 is a graph that indicates the simulated change in peak
wavelength as a function of viewing angle for one embodiment wherein
spacer 1 is air and spacer 2 is SiO2. In this example, the thickness
of spacer 1 is 3060 Å and the thickness of spacer 2 is SiO2 is
2060 Å. At a viewing angle of about zero degrees (corresponding to
viewing along axis 110 or 110' of FIG. 2A or FIG. 2B), the peak
wavelength reinforced by a first optical cavity formed with the air gap
(corresponding to area 130 in this example) is substantially the same as
the peak wavelength reinforced by a second optical cavity formed with
SiO2 (here, corresponding to area 135). In this example, the peak
wavelength at about zero degrees is about 650 nanometers (nm), which
corresponds to a red color. Other wavelengths are attenuated, to varying
degrees.

[0054]Accordingly, a human or machine observer with a normal ability for
color perception would perceive areas 130 and 135 as being a continuous
red area. (When a "human observer" or the like is referred to herein,
such expressions will reference a human observer with a normal ability
for color perception [e.g., not colorblind] unless otherwise indicated.)
Interface 145 would not be visible.

[0055]In this example, λAir is the peak wavelength for the
first optical cavity, whereas λd is the peak wavelength for
the second optical cavity. The size of the air gap in the first optical
cavity is dair. The index of refraction of the dielectric is
nd. Accordingly, λair is proportional to dair sin
θ. Similarly, λd is proportional to dair/nd
sin θ. When the viewing angle θ is zero, we want
λair to equal λd, as noted above.

[0056]Accordingly, the change in the peak wavelength for the first optical
cavity may be expressed as:

dλair∂dair cos θdθ. (Equation 1)

[0057]Similarly, the change in the peak wavelength for the second optical
cavity may be expressed as:

dλd∂dair/nd cos θdθ.
(Equation 2)

[0058]As the viewing angle increases, the peak wavelength passed by the
first optical cavity changes more rapidly than the peak wavelength passed
by the second optical cavity. At or beyond a threshold viewing angle, a
chromatic difference between the first and second optical cavities will
appear to the human observer.

[0059]When the viewing angle reaches 20 degrees, for example, the peak
wavelength for the first optical cavity is approximately 575 nm, which is
in the yellow range, whereas the peak wavelength for the second optical
cavity is about 620 nm, which is in the orange range. Some human viewers
may be able to detect this chromatic difference. At a viewing angle of 40
degrees, the peak wavelength for the first optical cavity is
approximately 380 nm, which may appear as a dark violet, whereas the peak
wavelength for the second optical cavity is about 490 nm, which is in the
blue/green range. At some viewing angle θ between 20 and 40
degrees, most human viewers would be able to detect a chromatic
difference between area 130 and 135, and would also detect a difference
in color at the interface 145.

[0060]A second example is provided by simulation results shown in the
graph of FIG. 4. Here, spacer 1 is made of AlOx and spacer 2 is
formed of SiO2. In this example, spacer 1 is approximately 340 nm
thick and spacer 2 is approximately 390 nm thick. Absorber 210 is formed
of MoCr and reflector 215 is made of Al.

[0061]In this example, the differences in the rates of color shift as a
function of viewing angle are not as great as those depicted in the
previous example. Nonetheless, when the viewing angle reaches 30 degrees,
the peak wavelength for the first optical cavity that includes the
AlOx spacer is slightly less than 600 nm, which is in the
yellow/orange range, whereas the peak wavelength for the second optical
cavity formed with SiO2 is about 570 nm, which is in the yellow
range. Some human viewers may be able to detect this chromatic
difference.

[0062]The apparent chromatic difference depends on the viewing distance
and/or size of the areas. For example, if areas 130 and 135 are as small
as subpixels and intermixed, then at an arm's length there may be an
overall color shift whereby the individual subpixels' color blends
together. However, if the areas are each large enough relative to the
viewing distance, e.g., 1'' squares at an arm's length, the chromatic
difference may be easily noticeable because the eye can resolve the
separate color blocks.

[0063]At a viewing angle of 40 degrees, the peak wavelength passed by the
first optical cavity spacer is approximately 545 nm, which is in the
green range. The peak wavelength passed by the second optical cavity is
about 490 nm, which is in the blue/green range. Other wavelengths are
attenuated. At some viewing angle θ between 30 and 40 degrees, many
human viewers would be able to detect a chromatic difference between area
130 and 135, and would also detect interface 145.

[0064]The rates of simulated color shift for a third example are shown in
FIG. 5. In this example, spacer 1 is approximately 203 nm thick and
spacer 2 is approximately 142 nm thick. Here, spacer 1 is made of
SiO2 and spacer 2 is made of Si3N4. Absorber 210 is formed
of MoCr and reflector 215 is made of Al.

[0065]In this example, the differential rate of color shift as a function
of viewing angle is somewhat greater than that depicted in the previous
example. When the viewing angle reaches 30 degrees, the peak wavelength
passed by a first optical cavity that includes the Si3N4 spacer
is approximately 620 nm, which is in the orange range, whereas the peak
wavelength passed by a second optical cavity formed with SiO2 is
about 570 nm, which is in the yellow range. Some human viewers may be
able to detect this chromatic difference.

[0066]At a viewing angle of 40 degrees, the peak wavelength passed by the
first optical cavity is slightly less than 600 nm, which is in the orange
range. The peak wavelength passed by the second optical cavity formed
with SiO2 is about 490 nm, which is in the blue/green range. This
chromatic difference should be easily detectable to a machine observer or
human with normal color perception. At some viewing angle θ between
30 and 40 degrees, most human viewers would be able to detect a chromatic
difference between area 130 and 135, and would detect a color change at
interface 145.

[0067]Some implementations provide hidden images that may not be
detectable by a human observer at any viewing angle. In some such
implementations, one or more of the peak wavelengths may be outside the
range of wavelengths that a human observer can perceive. Such
implementations may nonetheless be useful, e.g. if the image is
detectable by a machine device at some viewing angle.

[0068]One example is provided in FIGS. 6A and 6B. In this example, when
optical scanner 600 is in position 605, optical scanner 600 is oriented
to scan along axis 610. When optical scanner 600 is in position 615,
optical scanner 600 is oriented to scan along axis 620, which is at an
angle θ relative to axis 610. As in the previous examples, axis 610
is perpendicular to sheet 601, on which hidden image 625 has been formed.

[0069]When optical scanner 600 is oriented to scan along axis 610, hidden
image 625 is not readable by the optical scanner. (See 601a.) However,
when optical scanner 600 is oriented to scan along axis 620, areas 630
are distinguishable from areas 635. (See 601b.) Interfaces 645 between
areas 630 and 635 hidden image 625 can be detected. Accordingly, hidden
image 625 is readable by the optical scanner.

[0070]In this example, areas 630 and 635, along with interfaces 645,
define a bar code. Here, the peak wavelengths that are reinforced in
areas 630 and 635 are outside the range of wavelengths perceivable by a
human observer. Therefore, the bar code is not detectable by a human,
regardless of the orientation of sheet 601. In alternative
implementations, the peak wavelengths that are reinforced in areas 630
and 635 may be within the range of wavelengths perceivable by a human
observer.

[0071]Some methods of fabricating thin film stacks that include hidden
images will now be described with reference to FIGS. 7 through 10. As
with other examples described herein, there are a large number of
variations that are possible but yet still provide benefits described
herein. The steps of these methods are not necessarily performed in the
order indicated. Some implementations may involve additional steps,
whereas some implementations may omit indicated steps.

[0072]FIG. 7 depicts the steps of method 700, which outline some processes
of forming thin film stacks with hidden images on a substantially
transparent substrate. The substrate may be any suitable material, such
as glass, acrylic, plastic, etc. In step 701, the substrate is prepared
(e.g., cleaned and dried, if necessary) and positioned for the next step.

[0073]In this example, the substrate is a sheet of plastic and the steps
of method 700 are performed in the context of roll-to-roll processing,
including shadow masking or the like. However, a similar process may be
followed for other manufacturing techniques, e.g., those involving
separate substantially transparent substrates, such as sheets of glass,
separate sheets of plastic, etc. Some such techniques may involve other
types of manufacturing processes, such as embossing (e.g., to form
optical cavities in a substrate), semiconductor processing techniques
(such as etching, sputtering, chemical vapor deposition, etc.), selective
laser sintering methods, etc.

[0074]FIGS. 8A and 8B indicate simplified examples of forming thin film
stacks with hidden images by using roll-to-roll processing. In these
examples, only a single thin film stack and a single hidden image are
formed at one time. In practice, it is contemplated that many instances
of such stacks (and in some instances different types of stacks) would
generally be fabricated at a time.

[0075]Examples of the steps shown in FIG. 7 will first be provided by
reference to FIG. 8A, so both figures will be referenced in the following
discussion. Referring first to FIG. 8A, plastic sheet 801 is provided and
positioned by unrolling sheet 801 from roll 805. (Step 701.) An absorber
layer 810 is then formed on the substrate. (Step 705.) Here, absorber 810
is formed of MoCr. Other suitable absorber materials include Cr, Mo, Ti,
Ta, W, Al and Si, and combinations thereof. Some absorber thicknesses are
in the 40 Å to 200 Å range, although other absorber thicknesses
may be used.

[0076]Here, the absorber 810 is shown covering the surface of sheet 801.
In this example, absorber may be applied by PVD or sputtering of a MoCr
target, e.g., with a shadow mask to cause deposition only where so
desired and prevent deposition elsewhere. Alternatively, absorber
material may be applied to sheet 801 in a manner similar to that used by
computer printers. Here, absorber application device 815 includes a
plurality of nozzles, each of which may be configured to form precise
amounts of absorber 810 onto sheet 801. Alternative implementations apply
absorber 810 using techniques that are broadly similar to those employed
in thermal transfer printing, laser printing, etc.

[0077]Some implementations may involve applying absorber 810 to localized
areas of sheet 801, e.g., in order to reduce material costs. In some such
implementations, droplets of absorber 810 may carry a slight electrical
charge. The placement of absorber 810 on sheet 801 may be determined by
the charge of a cathode and electrode between which the absorber 810
moves towards sheet 801.

[0078]Spacer deposition modules 820 and 825 are used to deposit spacer
material (e.g., dielectric material) in first and second areas 130 and
135 (FIG. 1) of the absorber layer. (See steps 710 and 715 of FIG. 7.)
Here, a first dielectric material is applied to form spacer 1 in areas
130 and a second dielectric material is applied to form spacer 2 in areas
135. In this example, spacer 1 is made of SiO2 and spacer 2 is made
of Si3N4.

[0079]In this implementation, spacer 1 is formed in a substantially
uniform thickness of approximately 203 nm and spacer 2 is formed in a
substantially uniform thickness of approximately 142 nm. Here, spacer 1
and spacer 2 deposition modules 820 and 825, respectively, spray on the
dielectric material, allow it to harden and then remove excess material
(e.g. by wet or gas phase etching, or chemical-mechanical planarization
for relatively more durable substrates 801, e.g., glass substrates 801).
However, any suitable method for depositing dielectric material of a
desired type, planar orientation in the appropriate areas 130 and 135,
and thickness may be used.

[0080]Reflector applicator 830 then applies a reflective coating 835 to
the dielectric material. (See step 720 of FIG. 7.) Reflective coating 835
may be any of various reflective materials, e.g., a reflective metal such
as aluminum, gold, silver, dielectric mirrors, etc. Moreover, any
suitable method for applying reflective coating 835 may be used. For
example, a thin-film deposition method, such as an evaporation or
chemical or physical vapor deposition method, may be used to apply
reflective coating 835 to the dielectric material.

[0081]Step 725 of FIG. 7 involves any final processing steps that may be
required. For example, the stack may be rolled up with a plurality of
other stacks on sheet 801, may be cut or otherwise separated from the
rest of sheet 801, may be trimmed, packaged, cured, etc. The process ends
in step 730.

[0082]A similar roll-to-roll process is depicted in FIG. 8B. As before,
absorber 810 is applied to sheet 801. Spacer deposition modules 820 and
825 are used to deposit spacer material in first and second areas of the
absorber layer. Here, a first spacer is formed in areas 635 and a second
spacer is formed in areas 640. A reflector 835 is then applied.
Accordingly, the resulting stack includes hidden image 625, which is a
bar code in this example.

[0083]Alternative stacks and methods of fabrication are provided herein.
FIGS. 9A, 9B and 10 illustrate two such stacks and a corresponding method
of fabrication. Referring first to FIG. 9A, substrate 905 is first
prepared (see step 1001 of FIG. 10). The preparation process may involve
cleaning, drying, etching, embossing, etc. For example, an embossing
process or the like may be used to prepare recesses of a desired depth in
the substrate.

[0084]Then, reflector 915 is deposited onto substrate 905. (See step 1005
of FIG. 10.) Step 1005 may involve applying a continuous reflector layer
915, as shown in FIG. 9A. However, in some implementations, step 1005 may
involve applying a discontinuous reflector and/or applying reflector
material only in certain portions of the substrate. Alternatively, the
substrate 905 may have sufficient reflectivity to act as reflector 915,
for example a polished aluminum substrate 905.

[0085]According to this implementation, a spacer 1 and spacer 2 are
deposited on first and second areas 130 and 135 of the reflector layer.
(See steps 1010 and 1015 of FIG. 10.) Spacer 1 and spacer 2 of FIG. 9A
are substantially the same thickness. However, as described elsewhere
herein, the spacers may be formed into any desired thickness and need not
have the same thickness. Moreover, more than two spacers and/or spacer
types may be formed.

[0086]Absorber layer 910 is then formed on spacer 1 and spacer 2. (See
step 1020 of FIG. 10.) Then, a substantially transparent layer 920 is
formed on absorber layer 910. (See step 1025 of FIG. 10.) After the final
processing steps, if any (see step 1030 of FIG. 10), optical stack 900 is
formed. In this example, the thickness and refractive index of spacer 1
and spacer 2 are selected such that the same peak wavelength of reflected
light is passed by both optical cavities when viewed from position 925,
along axis 930 that is substantially perpendicular to reflector layer
915. However, a chromatic difference may be perceived when viewed from
position 935, along axis 940 that is at an angle θ relative to axis
930.

[0087]In the example depicted in FIG. 9B, the same spacer material is used
in optical cavities 955, 960 and 965. In this example, the spacer
material is SiO2. These optical cavities include the spacer
material, absorber 910 and reflectors 915. Absorber 910 is proximate
substantially transparent substrate 920.

[0088]In this example, cavity 960 includes a much thicker spacer than
cavities 955 and 965. Here, the thickness of cavity 960 causes reflected
light to be resonant in the visible range, whether viewed from position
925 along axis 930 normal to reflector 915 or from position 935 along
axis 940 that is θ degrees from axis 930.

[0089]In this example, the SiO2 spacer material in cavity 960 is 760
nm thick. This causes peak wavelengths of approximately 458 nm (blue) and
571 nm (yellow) to be passed by optical cavity 960 when viewed along axis
930, corresponding to white at approximately (x=0.30, y=0.31) in the
standard CIE xyY color space. When viewed along axis 940 with 0=20
degrees, cavity 960 passes peak wavelengths of approximately 421 nm
(violet), 522 nm (green) and 693 nm (far red), corresponding to green at
approximately (0.21, 0.44). When viewed along axis 940 with 0=34 degrees,
cavity 960 passes peak wavelengths of approximately 468 nm (blue) and 621
nm (red), corresponding to magenta at approximately (0.37, 0.22).

[0090]However, the thicknesses of cavities 955 and 965 have been selected
such that cavities 955 and 965 are not resonant in the visible range,
whether viewed from position 925' along axis 930' normal to reflector 915
or from position 935' along axis 940' that is 0 degrees from axis 930'.
When viewed along axis 930', cavities 955 and 965 pass wavelengths in a
broad spectral range, corresponding to white at approximately (0.332,
0.34). When viewed axis 940 with θ=45 degrees, cavities 955 and 965
still pass wavelengths in a broad spectral range, corresponding to white
at approximately (0.329, 0.34).

[0091]Therefore, the light reflected from cavities 955 and 965 appears to
be white and does not color shift nearly as much as light reflected from
cavity 960. A colored image from cavity 960 would seem to appear out of a
white background from cavities 955 and 965 as the viewing angle
increases.

[0092]Although many of the components and processes are described above in
the singular for convenience, it is contemplated that multiple components
and repeated processes can also be used to practice that described
herein. Similarly, although illustrative embodiments and applications of
this invention are shown and described herein, many variations and
modifications are possible which remain within the concept, scope, and
spirit of the invention, and these variations would become clear after
perusal of this application.

[0093]For example, in some implementations an image may be apparent when
viewed from normal incidence, but no image is apparent when viewed from a
sufficiently large angle. Such alternative implementations may involve
optical cavities configured to pass noticeably different wavelengths when
viewed along an axis perpendicular to a surface (e.g., perpendicular to a
reflective surface of a cavity) and to pass substantially the same
wavelength when viewed from an angle relative to that axis. Accordingly,
the present embodiments are to be considered as illustrative and not
restrictive, and the invention is not to be limited to the details given
herein, but may be modified within the scope and equivalents of the
appended claims.